The present invention relates to a magnetic tape memory apparatus of helical scan scheme for recording digital data, and in particular to such a scheme suitable for rewriting data already recorded.
A magnetic tape apparatus of helical scan type provides a higher recording density than that of a magnetic tape apparatus of fixed head type. Therefore, magnetic tape apparatuses of helical scan type are widely used in fields needing a large quantity of information such as the VTR field. Because of advancement of the VTR technique in recent years, domestic VTRs are produced in large quantities and at reasonable costs and are popularized significantly. Further there is a digital audio tape recorder system (R-DAT) for recording/reproducing PCM signals of music onto/from tape stored in a cassette case. In this DAT, digital signals are recorded. The tape cassette is as small as 73 mm (width).times.54 mm (depth) .times.10.5 mm (height) and permits stereophonic sound recording for up to two hours. And the tape cassette has a capacity of approximately 1.4 Gbyte when converted in bytes. The DAT has a search function capable of reproducing and gaining access to tape while running the tape at high speed. Thus the DAT is suitable to use as external memory apparatuses. By using this DAT technique, it is possible to realize an inexpensive magnetic tape memory apparatus having a large capacity. The DAT will now be described. FIG. 1 is a schematic configuration diagram of a tape traveling system. In FIG. 1, numeral 1 denotes a rotating drum, 2a and 2b magnetic heads attached to the drum 1 differing from each other in azimuth angle by .+-.20 degrees, 3 magnetic tape, 4a, 4b and 4c respectively guide posts, a capstan and a pinch roller included in tape traveling means for winding the tape 3 around the drum 1 and traveling the tape 3, and 5 a tape cassette. The tape 3 is 3.81 mm in width and 13 .mu.m in thickness. With respect to standards, there are several modes depending upon the sampling frequency of A-D conversion, the number of bits per word and the number of channels. However, a standard mode will now be described. The tape 3 is driven by the capstan 4b and the pinch roller 4c at a traveling speed of 8.15 mm/sec. The drum 1 is rotated at 2,000 rpm. While the tape 3 is slantly wound around the drum 1 over a 90 degree range with a lead angle of 22 minutes and 6 degrees to travel by the traveling means 4a, signals are recorded onto/reproduced from the tape by the magnetic heads 2a and 2b. A pattern on recording tracks thus recorded on the tape is shown in FIG. 2. In FIG. 2, numeral 6 denotes a track recorded by the magnetic head 2a and numeral 7 denotes a track recorded by the magnetic head 2b. The track pitch P, the track length L and the recording width W.sub.R are 13.591 .mu.m, 23.501 mm and 2.613 mm, respectively. FIG. 3A shows the recording format on the magnetic tape. One track has 196 blocks. PCM signals comprising 16 bits per word are divided into 8-bit groups (symbols) and recorded. As shown in FIG. 3B, one block includes a datum occupying 32 symbols, a synchronization signal (SYNC) occupying one symbol, an ID code occupying one symbol, a block address occupying one symbol and a parity code occupying one symbol for detecting errors caused in the ID code and the block address. That is to say, one block includes 36 symbols in total. The ID code is a signal for discriminating the sampling frequency, the number of channels and the like of the PCM signal. In one track, 128 blocks of the PCM signal are arranged in a control portion having a tape winding angle of approximately 60 degrees. On both sides of the central portion, a region for recording an ATF signal used for tracking control is provided. Further, on both sides of the ATF signal region, a subcode having 16 blocks is disposed so as to have 8 blocks on each side. In the subcode region, sequence of program, time and the like are recorded. The subcode region has the same block configuration as that of the PCM signal region.
Reliability of data is a very important factor in memory apparatuses of computers. When data are to be recorded in a flexible apparatus or a fixed magnetic disk apparatus, therefore, data to be recorded are first recorded onto a magnetic disk, and the data are then reproduced to confirm that the data are correctly recorded. If there is an error, the data are recorded again or recorded in an alternate sector to perform verification operation. In order to improve the reliability of data, error correction codes are used in the DAT and the PCM recorder. Even if an error which cannot be corrected is caused at a rate of, say, approximately once every 30 minutes, it cannot be said that this error is fatal. Therefore, error correction codes alone suffice and it is not necessary to confirm the recorded data. In case of use as an external memory apparatus of a computer, however, any minute error may be fatal. For avoiding a defect on the magnetic tape and false recording due to an invasion of a foreign substance, therefore, it is indispensable to check whether data are correctly recorded. Further, it is necessary to partially correct data already recorded and write the corrected data at the same location as that of the original data and link data together afterwards to record them. Examples relating to checking whether recording has been correctly performed in the helical scan type and after recording are described in JP-A-58-122606, JP-A-61-139909, and JP-A-59-195306.
In the helical scan type, the magnetic tape travels at a constant velocity. A recording track is formed by adding the velocity vector of the rotating drum to that of the tape. In case continuous recording operation is performed in the conventional magnetic head configuration, the magnetic head automatically moves to the next track, and hence it is not possible to reproduce and confirm the recorded data. It is not desirable to drive the tape intermittently because it not only casts a heavy burden upon the tape drive and control apparatus but also requires start time and stop time. Further, it is also possible to rewind the magnetic tape and then travel the tape again to reproduce data for the purpose of reproducing and confirming the data recorded once. However, it takes a long time to rewind the tape and restart traveling and hence the data confirmation in such a form is not practical. The verifying function is attained by using a composite head comprising a recording head and a reproducing head united in one body and by reproducing immediately after the data recording to compare the recorded contents with the reproduced contents. Since the reproduced signal is weak, however, it is difficult to reproduce data while recording data because of a crosstalk problem.
Therefore, it is necessary to make the distance angle between the recording head and the reproducing head equal to or larger than the recording angle onto the tape (corresponding to the winding angle). In the DAT, azimuth recording is performed. That is to say, data are alternatively recorded on tracks without providing a guard band. In general, the recording width (corresponding to the gap width W.sub.1) of the magnetic head is larger than the track pitch P. In the reproduction operation, the amount of off-track is detected by using the above described ATF signal, and a negative feedback closed loop is so configured to control the tape traveling that the center of the gap of the reproducing head in the width direction of the gap may come to the center of the recording track. Since the gap width W.sub.1 of the magnetic head is larger than the track pitch P, however, writing operation of data onto one track shaves its preceding and succeeding tracks. Once data have been recorded, therefore, it is difficult to rewrite only one track or only one portion within one track. If a group is constituted by a plurality of tracks and data are rewritten while taking one group as unit, such a problem is solved. If the ATF signal region for tracking control is also rewritten, however, there is a problem that adjacent groups might be eroded in rewriting data many times because of difference between the tape velocity and the rotation velocity of the cylinder and expansion caused by the temperature difference.